The present invention relates to a process for the condensation of solids, particularly phosphates, which tend to sinter and/or adhere during condensation when heated in a reactor.
Processes of this type include the condensation of acidic alkali, alkaline earth and heavy metal monophosphates or ammonium metal monophosphates to the corresponding polyphosphates, for example the condensation of monosodium monophosphate to disodium diphosphate and further to Madrell's salt or trimetaphosphate; the condensation of disodium monophosphate to tetrasodium diphosphate; the condensation of lithium monophosphate to the corresponding di- and polyphosphates, and the condensation of monocalcium and monomagnesium monophosphates through the diphosphates to the corresponding polyphosphates.
The following may be cited as examples of the conversion of heavy metal monophosphates:
The production of aluminum tripolyphosphate or aluminum metaphosphate from aluminum monophosphate, and the condensation of zinc, manganese or copper diphosphate from the corresponding monophosphates.
Further examples are the manufacture of boron phosphate by condensing boric acid with ammonium phosphate, the formation of ammonium polyphosphate from urea phosphate, or the production of melamine polyphosphates.
In the condensation of urea to cyanuric acid and in the formation of phosphorus nitride oxide or phosphorus nitride sulfide, the adhesion of the products causes significant difficulties in that the final product adheres to the walls of the reactor and interferes with the transfer of heat, thereby decisively affecting the degree of conversion.
A method and an apparatus for carrying out high temperature processes at temperatures in excess of 300.degree. C. is disclosed in DE-OS No. 15 42 016, wherein in a screw extruder with one or more worm shafts the initial product is at first heated overwhelmingly by the internal mechanical friction to a degree sufficient to transform it through a viscous, paste-like state, to a mass of reduced viscosity which is then supplied with the necessary heat by means of direct heat transfer. Such devices are in principle unsuitable for carrying out condensation reactions because the mixing effect is inadequate so that the conversion remains incomplete.
Further, mixing machines are disclosed in DE-OS No. 15 57 119 and DE-OS No. 20 12 294, which are provided with cylindrical or trough-like housings equipped with agitators and kneading bars and/or disk-like mixing elements. Only processes in which the temperature does not exceed approximately 350.degree. C. can be carried out in mixers of this type.